Power supply for identification and control of electrical surgical tools

ABSTRACT

An electronic circuit for identifying an electrical surgical tool and for providing a selectable constant current appropriate to the identified electrical surgical tool.

This application is continuation-in-part of U.S. application Ser. No.10/099,500 filed on Mar. 13, 2002 now U.S. Pat. No. 6,695,837.

FIELD OF THE INVENTIONS

The devices described below relate to power supplies intended to supplyelectrical power to medical instruments.

BACKGROUND OF THE INVENTIONS

Many electrical surgical devices are provided in the form of electricalsurgical tools, such as a thermal cautery device, which can be pluggedinto a separate power supply. Typically, the power supplied to theelectrical surgical tool must be carefully controlled; thus, the powersupply includes circuitry to convert available AC power to AC, RF or DCpower at the desired output power levels or frequencies. For example,Herzon, Thermal Cautery Surgical Forceps, U.S. Pat. No. 6,235,027 (May22, 2001), shows thermal cautery forceps using a power supply to delivera regulated current to the resistive heating elements in the forceps.Our own cautery instruments, such as the Starion® Thermal CauteryForceps, which comprise forceps with resistive heating elements disposedon the grasping tips, are designed to work with our PowerPack SurgicalPower Supply. Currently marketed versions of this power supply provide acurrent to the resistive heating elements depending on the heat load andtemperature of the resistive heating device. In addition to these twodevices, many electrical surgical instruments are currently marketed toaddress a variety of surgical techniques and the number of surgicalinstruments available has been growing.

The increase in the variety of surgical instruments has introduced aproblem in medical-grade power supplies. Most power supplies can operatewith different kinds of medical instruments, as long as an electricalconnection can be established between the power supply and theinstrument. However, a medical device manufactured by one company mayperform slightly differently than expected when the medical device isused in conjunction with a power supply from another company. Since manymedical procedures require precise control of the electrical propertiesof the medical device, a surgeon or doctor may unintentionally harm apatient when the surgeon uses a power supply and a medical device fromdifferent manufacturers. For example, though the Starion® PowerPackprovides optimal power to the various Starion® electrical surgical toolsfor which it is intended, the use of connectors available to othermedical device manufacturers may permit use of non-Starion® electricalsurgical tools with the PowerPack. When used in combination with suchthird party electrical surgical tools, it is not possible to ensure thatthe optimal amount of power is delivered to the tool. Thus, the tool maynot function as desired, with the result that the patient may be harmed.Thus, a medical-grade power supply is needed which operates only withthe instruments made by that manufacturer and tested with that model ofpower supply.

SUMMARY

The methods and devices described below relate to a power supply thatidentifies an electrical surgical tool, such as a thermal cauterydevice, and provides power only to electrical surgical tools that areidentified by the power supply. The power supply uses a deviceidentification circuit and a constant current circuit to control thepower output to an electrical surgical tool. The device identificationcircuit identifies whether an electrical surgical tool is designed to beused with that power supply. If the device identification circuitrecognizes the electrical surgical tool, then the constant currentcircuit will provide a constant current, or electrical power, to thetool. In addition, the constant current circuit will provide the toolwith the amount of power required by that particular device. If thedevice identification circuit does not recognize the device connected tothe power supply, then the constant current circuit provides no power tothe device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the physical power supply box.

FIG. 2 is a block diagram of the power supply system.

FIG. 3 is a block diagram of the power board system.

FIG. 4 is a circuit diagram of the device identification circuit.

FIG. 5 is a device identification table showing specific values of theidentification resistor, the identification voltage and the comparatoroutputs for three distinct thermal cautery devices and a foreign device.

FIG. 6 is a circuit diagram of the constant current circuit.

FIG. 7 is a circuit diagram of an alternate device identificationcircuit.

FIG. 8 is a circuit diagram of an alternate device identificationcircuit with switching.

DETAILED DESCRIPTION OF THE INVENTIONS

FIG. 1 illustrates a physical power supply box 1. The power supply box 1is typically a non-sterile, reusable, AC powered device designed for useonly with certain instruments, medical devices, electrosurgical devicesor other electrical surgical tools such as thermal cautery devices. Thepower supply is connected to an AC power source. The power supply has anon-off switch 2, an AC power connector 3, an LED power indicator 4, acontrol knob 5 and a hanger 6 suitable for suspending the power supplyon a support object, such as an IV pole. The power supply is alsoequipped with an input 7 for an interface board, control board orswitches and an output 8 to the medical instrument. The supply has inputrequirements of 100 to 240 VAC at 50 to 60 Hz and at 80 W, has a maximumoutput of 32 VA, a no load voltage of 5V DC, and is operated at a dutycycle of about 5 seconds on and 10 seconds off. The output current is inthe range of 2.4 A to 4.4 A, depending on which instrument is used andon the output desired by the user.

One version of the power supply, which is intended for use with cauteryinstruments, has three output heat levels: low, medium and high. Thelevels correspond to the desired heat output of the thermal cauteryinstrument's resistive heating element or elements, and correspond tocurrent outputs of the power supply. A tone indicates the level of heatbeing applied to the cautery device. For example, a low tone indicates alow heat setting, an interrupted low tone indicates a medium heatsetting and high tone indicates a high heat setting. Other devices havemultiple heating elements and each element may have different heatinglevels. For medical devices with multiple settings or elements, thevarious settings and elements are adjustable. The adjustment of thesesettings or elements is typically facilitated by an interface board, orcontrol board, that can switch between power levels or heating elements.

FIGS. 2 and 3 illustrate embodiments of the thermal cautery device,power supply and control system. The control system comprises the deviceidentification circuit, the constant current circuit and, if used, thecontrol voltage circuit and/or the user interface board. The controlsystem thus comprises a means for connecting the switching power supplyto the resistive heating element to provide power to the resistiveheating element.

FIG. 2 is a block diagram of the power supply system and control system.Block 11 is the AC power input and is equipped with a power on/offswitch 13. The AC power is connected to an isolated switching DC powersupply 12, or AC to DC converter, through switch 13. However, the AC toDC converter can be replaced with an AC to low frequency AC converter,an AC to high frequency AC converter or an AC to low power AC converter.The isolated switching DC power supply 12 typically has an output of +5volts DC. In addition, the isolated switching DC power supply is ULrated for heart contact, meaning that the output of the power supply candirectly touch the patient. The DC power supply 12 is connected to apower board 14 which contains a constant current circuit, a controlvoltage circuit and a device identification circuit. The power board 14is also connected to the electrical surgical tool 15, a power LED 16indicating whether the power supply is on and an interface or controlboard 17. A user may provide input through the interface board to affectthe settings of the power board and hence affect the operation of thethermal cautery device. The interface board 17 is optionally connectedto a switch module 18, which typically has pedal or push buttonswitches. The output of the switch module allows the user to select thelevel of power desired for a given procedure, or to select other modesof operation available to particular medical devices, when the switchmodule is connected to the control system. Alternatively, the switchescan directly control the power level provided to the medical device. Theinterface board 17 is also attached to a current control knob or switch19, a device LED 20 indicating whether an attached device is capable offunctioning with the power supply, and a switch LED 21 that indicatesthat use of the switch module 18 is required. Note that other interfaceboards may be added to the power board.

FIG. 3 is a block diagram of the power board system. The power board 14is conceptualized as three circuits: a device identification circuit 26,a control voltage circuit 27 and a constant current circuit 28. Theelectrical surgical device 15 is electrically connected to the deviceidentification circuit 26 and receives current from the constant currentcircuit 28. A constant DC power source, V_(cc) (item 29), is provided toall three circuits 27, 28 and 29, and is operated to supply power atabout +5 volts DC. Note that other circuit blocks may be added, such asa circuit that provides a tone indicating the level of power runningthrough the medical device or a circuit that facilitates the use ofadditional switches or pedals to control the power output of the powersupply.

FIG. 4 is a circuit diagram of the device identification circuit 26,which includes all three diagrams shown in FIG. 4. The portion of thedevice identification circuit shown in area 35 (comprising the box 35shown in phantom) contains the portion of the identification circuitlocated in the electrical surgical tool. The portion of the deviceidentification circuit shown in area 35 comprises an identifyingelectrical component. In the embodiment shown in FIG. 4 the identifyingelectrical component is a resistor, though a capacitor, an inductor orother electrical component capable of uniquely identifying the cauterydevice may be used as the identifying electrical component. (In the caseof a capacitor or an inductor, the device identification circuit wouldbe modified to use the chosen identifying electrical component.) Theportion of the identification circuit shown in area 36 (comprising thebox 36 shown in phantom) is located in the power supply box. The variouscomponents of the device identification circuit can be placed in eitherthe electrical surgical device or the power supply box. For example, allof the circuit could be inside the electrical surgical tool or all ofthe circuit could be placed inside the power supply box. However, if theidentification resistor, R_(id), is placed inside the power supply box,then means are provided such that a specific electrical surgical deviceplugs into a corresponding identification resistor. For example, adifferent outlet in the power supply box can be provided for eachelectrical surgical device. Alternatively, each electrical surgicaldevice plug can have a different pin arrangement that plugs into asingle outlet in the power supply box. In this case, the pin arrangementselects the proper identification resistor.

The device identification circuit can determine whether an electricalsurgical device is plugged in or plugged in properly, whether the deviceis a device for which the power supply is designed, and which of aplurality of electrical surgical devices designed for use with the powersupply is electrically connected to the power supply. In addition, thecircuit of FIG. 4 constitutes a device identification means (or anelectrical surgical device identification means or a thermal cauterydevice identification means), though the circuit may be varied in manyrespects. For example, the circuit of FIG. 4 is designed to identifythree medical devices, though the circuit could add additional resistorsto the resistor ladder and add additional comparators so that the deviceidentification circuit can identify a plurality of electrical surgicaldevices or other kinds of electrical medical devices. In addition,capacitors can be added or subtracted from the circuit in order tocreate different kinds of filters. Also, the resistive heating elementof the electrical surgical device can comprise the identificationresistor as long as the resistive heating element of each electricalsurgical device has a distinct resistance.

Referring again to FIG. 4, a voltage V_(cc) is placed across a resistor,R_(id), which is located in the plug of the medical device, and areference resistor, R_(r), connects to ground. An identifying voltage,V_(id), develops across R_(r). Note that V_(id) varies with the value ofR_(id). R_(id) is set by the manufacturer and is unique to a particularmodel of medical device. A capacitor, C_(r), is placed in parallel withthe reference resistor and operates as a low pass filter.

The identifying voltage, V_(id), is sent to a comparator, which comparesV_(id) to a reference voltage, V_(r). If the identifying voltage,V_(id), is greater than the reference voltage, V_(r), then thecomparator (which can be a true comparator or an operational amplifieroperated as a comparator) outputs a “1” signal. If the identifyingvoltage is less than the reference voltage, then the comparator outputsa “0” signal. The output of the comparator is provided to the controlvoltage circuit 27, which generates a control voltage, V_(c). Thecontrol voltage determines, through the constant current circuit 28, theamount of power provided to the medical device.

The device identification circuit in FIG. 4 is designed to detect threekinds of medical instruments, each of which has a separate identifyingresistor, R_(id). Thus, the voltage cascade circuit has four comparisonresistors, R_(c1), R_(c2), R_(c3) and R_(c4), placed in a resistorladder. The resistor ladder produces a voltage cascade comprising aseries of reference voltages. A reference voltage is taken between eachof the reference resistors, the reference voltages comprising V_(r1),V_(r2) and V_(r3). Each reference voltage is provided to the negativeterminal of a distinct comparator; thus, V_(r1) is provided tocomparator 37, V_(r2) is provided to comparator 38, and V_(r3) isprovided to comparator 39. On the other hand, the same identifyingvoltage, V_(id), is provided to the positive terminal of eachcomparator. The output of each comparator, 40, 41 and 42, is provided tothe control voltage circuit 27.

A first model of a medical instrument (device A) has an identifyingresistor, R_(id), with the smallest value. In this case the identifyingvoltage, V_(id), will be higher than all three of the referencevoltages. Thus, all three comparators will output a “1” signal. The factthat all three comparators output a signal is communicated through thecontrol voltage circuit 27, which outputs a control voltage, V_(c). Theconstant current circuit 28 then uses the control voltage to controlelectrical power to device A in an amount appropriate to device A.

Similarly, a second model of medical instrument (device B) will have anidentifying resistor, R_(id), of medium resistance. In this case theidentifying voltage, V_(id), will be lower than the first referencevoltage, V_(r1), but higher than the other two, V_(r2) and V_(r3). Thus,only comparators 38 and 39 will produce a “1” output. Accordingly, theconstant current circuit 28 will recognize that device B is connected tothe power supply. On the other hand, if a third model of medicalinstrument (device C) is connected to the power supply then V_(id) willbe less than V_(r1) and V_(r2), but greater than V_(r3). In this case,only comparator 39 will report a “1” output and the constant currentcircuit 28 will recognize that device C is connected to the powersupply. However, if R_(id) is not present or does not have the correctvalue, then all of the comparators will output a “0” signal. In thiscase the control voltage will be “0” and then the constant currentcircuit will provide no power to the device. Thus, the power supplyusing the device identification circuit of FIG. 4 will only work withthermal cautery devices A, B and C.

The various values of C_(r), R_(r), R_(id), R_(c), V_(r) and V_(id) areset by the manufacturer and can have a wide range of values. In oneembodiment V_(cc)=+5V, C_(r)=10 μF, R_(r)=10 KΩ, R_(c1)=15 KΩ, R_(c2)=20KΩ, R_(c3)=10 KΩ and R_(c4)=5.1 KΩ. In this case V_(r1)=3.5V,V_(r2)=1.5V and V_(r3)=0.5V. In addition, there are three thermalcautery devices designed by the manufacturer to operate with the powersupply, the thermal cautery devices having R_(id) values of 1 KΩ, 10 KΩand 51 KΩ respectively. Furthermore, V_(id) will have 3 differentvalues, one for each thermal cautery device, as shown in the table ofFIG. 5.

FIG. 5 is a device identification table showing the specific values ofthe identification resistor, the identification voltage and thecomparator outputs for the three distinct thermal cautery devices and aforeign device. The columns in FIG. 5 reflecting comparator outputs 40,41 and 42 show that each thermal cautery device has a unique set ofcomparator outputs. In the comparator output columns of FIG. 5, a “1”indicates an output signal of “1” and a “0” indicates an output signalof “0”. Note that for each device (table row) V_(r1)=3.5V, V_(r2)=1.5Vand V_(r3)=0.5V

The table of FIG. 5 shows that when thermal cautery device A, with anR_(id) of 1 KΩ, is plugged into the power supply then V_(id) is 4.5V,which is higher than all three of the reference voltages, V_(r1) (3.5V),V_(r2) (1.5V) and V_(r3) (0.5V). Thus, comparators 37, 38 and 39 alloutput a “1” signal and the system then knows that thermal cauterydevice A is plugged into the power supply. Applying similar logic, thesystem can tell if thermal cautery devices B or C are plugged into thesystem. However, if a foreign device is plugged into the system, or ifno device is plugged into the system, then R_(id) is infinity and thusV_(id)=0. If V_(id) is 0 then V_(id) is less than all three values ofV_(r); accordingly, all of the comparators will output a “0” signal andthe constant current circuit 28 will provide no electrical power to thethermal cautery device.

The output of the device identification circuit, 40, 41 and 42, is fedinto the input of the control voltage circuit 27. The control voltagecircuit is a series of logic gates and analog circuits connected to thecomparators and, optionally, to switches in the interface board. Thelogic gates, analog circuits and switches constitute a control voltagemeans. The control voltage circuit 27 outputs a control voltage, V_(c),based on the output of the comparators and, optionally, based on theinterface board switches 18. The control voltage that is output by thecontrol voltage means is unique to a particular electrical surgicaldevice that has been designed to work with the power supply. Theswitches and the control voltage circuit allow the user to select acontrol voltage from a set number of control voltages. The constantcurrent circuit 28 uses the control voltage, V_(c), to determine theamount of current delivered to the thermal cautery device.

FIG. 6 is a circuit diagram of the constant current circuit 28 which isoperable to provide a constant current to a thermal cautery device. Themain current path proceeds from V_(cc) (typically +5 volts), throughoutput plug (between terminals 46 and 47 when the terminals areelectrically connected), through a power MOSFET 48 and finally through asense resistor, R_(s), used to sense the amount of current, I_(main),flowing through the main current path. Other kinds of transistors canalso be used, such as a JFET transistor or an NPN transistor (BJTtransistor). A capacitor, 49, is placed between the terminals of theoutput plug and acts as a low pass filter. When current flows throughthe main current path, R_(s) generates a sense voltage, V_(s),proportional to the main current (V_(s)=I_(main)×R_(s)) The circuitprovides the sense voltage, V_(s), to the negative terminal of anoperational amplifier 50 through a low pass filter comprising resistor51 and capacitors 52 and 53. The operational amplifier 50 is alsoconnected to ground and to V_(cc). The positive terminal of theoperational amplifier is connected to the control voltage, V_(c),through a low pass filter comprising capacitor 54. The operationalamplifier compares the control voltage, V_(c), to the sense voltage,V_(s), and generates an output that controls the gate 55 of the powerMOSFET 48. (The MOSFET drain 56 and source 57 are shown forconvenience). The result is that the operational amplifier 50 adjuststhe current flowing through the power MOSFET 48 until the sense voltageis equal to the control voltage (V_(s)=V_(c)). (The control voltage isbased on the value of the identification resistor or other electricalidentification component.) Thus, the constant current circuit 28maintains the current at a steady level. The amount of current is set bythe value of the sense resistor, R_(s). For example, if the senseresistor, R_(s)=0.1 Ω and the control voltage, V_(c)=0.1V, then the maincurrent, I_(main)=1.0 A.

The output plug (terminals 46 and 47) is electrically connected to themedical instrument's resistive heating elements, thus completing theelectrical circuit. Accordingly, the current flowing through the maincurrent path powers the medical instrument reliably and selectably.

The circuit shown in FIG. 6 can be varied in many respects. For example,the capacitors and resistors can take on different values, may be takenout of the circuit or other capacitors and resistors may be added toaccomplish different filtering effects. In addition, the circuit can besimilarly modified to provide different output currents for a givencontrol voltage.

FIG. 7 is a circuit diagram of an alternate device identificationcircuit. A voltage, V_(cc), is placed across a resistor, R_(id), whichis located in the plug of the electrical surgical tool. R_(id) is set bythe manufacturer and is unique to a particular thermal cautery device. Asecond resistor, 63, is connected to a control resistor, 64, whichitself connects to ground. A control voltage, V_(c), develops acrosscontrol resistor 64 and V_(c) varies with the value of R_(id). Thecontrol voltage is provided to a constant current circuit, such as thatshown in FIG. 5, which provides a constant current to the thermalcautery device. The output current is a function of V_(c) and R_(id)(I_(output)=V_(c)/R_(id)).

FIG. 8 is a circuit diagram of an alternate device identificationcircuit with switching. Three circuits, similar to the one in FIG. 7,are combined using three different identification resistors, R_(id1),R_(id2) and R_(id3), though resistors 63 and 64 have the same value inall three circuits. In addition, a switch 65 is provided in eachindividual circuit between the second resistor 63 and the controlresistor 64. The switch 65 can be any kind of switch, such as a pushbutton, a pedal or a knob. Selecting a switch chooses one of the controlvoltages, V_(c1), V_(c2) or V_(c3) and sends that control voltage to aconstant current circuit, such as the one shown in FIG. 6. The constantcurrent circuit then provides a constant current to the electricalsurgical tool appropriate to the particular thermal cautery device andappropriate to the switch or switch combination selected. In oneembodiment the values of the components are R_(id1)=2.75 KΩ,R_(id2)=3.67 KΩ, R_(id3)=5.5 KΩ, R₆₃=10 KΩ, R₆₄=1 KΩ. Since V=IR then inthis embodiment I_(output)=11 KΩ/R_(id).

Depending on the design of the constant current circuit, control voltagecircuit or other additional circuit, the combination of switches cancontrol a variety of variables. For example, an electrical surgical toolmay have one subcomponent that requires power. In this case the threeswitches control whether the device receives a high, medium or lowamount of power. The constant current circuit shown in FIG. 6 canoperate with the device identification circuit of FIG. 8 to perform thisfunction.

Alternatively, other control voltage circuits can add additionalcapabilities when used in conjunction with the switch design of FIG. 8.For example, an electrical surgical tool can have three subcomponents,each of which require power. The three switches determine which, if any,of the subcomponents receive power. For other constant current circuitsit is possible to use different combinations of open and closedswitches. For the circuit shown in FIG. 8 there are eight possibleswitch combinations. Each combination of switch positions generates adistinct control voltage that can control different elements or powerlevels of the electrical surgical tool.

A plurality of switch circuits is also possible. In the case of aelectrical surgical tool with three powered subcomponents, each of whichhaving three power settings, then nine switches can control whichsubcomponent receives a given amount of power (depending on the designof the constant current circuit and control voltage circuits). Inaddition, a plurality of electrical surgical tools can be attached tothe circuit, each electrical surgical tool having a unique identifyingresistor. In this case the plurality of switches determine which, ifany, of the devices are on. Finally, a plurality of switches can beprovided on a circuit that accommodates multiple devices, each devicehaving multiple powered subcomponents, each powered subcomponent havingmultiple power levels, and wherein different combinations of switchpositions control different aspects of the device.

Although the methods, devices and circuits are described in relation toelectrical surgical tools, the same methods, devices and circuits can beused with other kinds electrical devices where device identification isdesired. For example, electrical surgical tools using DC, AC or RF powercan use the device identification methods described above. Electricalsurgical tools useable with the device identification circuit alsoinclude ablation devices, thermal ligation devices, thermal cauterydevices, electrocautery devices and other kinds electro-medicalinstruments. Thus, while the preferred embodiments of the devices andmethods have been described in reference to the environment in whichthey were developed, they are merely illustrative of the principles ofthe inventions. Other embodiments and configurations may be devisedwithout departing from the spirit of the inventions and the scope of theappended claims.

1. A system for performing thermal cautery system on body tissue, saidsystem comprising: a thermal cautery device, said thermal cautery devicecomprising a resistive heating element mounted on means for pressing theresistive heating element into contact with body tissue; a switchingpower supply; and a control system comprising a means for operablyconnecting the switching power supply to the resistive heating elementto provide power to the resistive heating element; and a means foridentifying the thermal cautery device connected to the switching powersupply, wherein the control system controls operation of the thermalcautery device based on the identity of the thermal cautery device; andwherein the means for identifying the thermal cautery device comprises:an identification resistor electrically connected to the thermal cauterydevice, said identification resistor having a pre-determined resistancevalue; and a reference resistor placed in series with the identificationresistor when the thermal cautery device is electrically connected tothe switching power supply, said reference resistor also electricallyconnected to ground such that an identifying voltage develops across thereference resistor when power is applied to the reference resistor andthe identification resistor; and a comparison resistor electricallyconnected to the switching power supply and to ground, wherein areference voltage develops across the comparison resistor when power isapplied to the comparison resistor; and a comparator, wherein thepositive terminal of the comparator is electrically connected to theidentifying voltage and the negative terminal of the comparator iselectrically connected to the reference voltage; wherein the comparatorproduces an output signal if the identifying voltage is higher than thereference voltage, said output signal indicating that a particularthermal cautery device is operably connected to the power supply.
 2. Thesystem of claim 1 wherein the thermal cautery device further comprisesan identifying electrical component having a predetermined value and thecontrol system further comprises a means for identifying the thermalcautery device based on the identifying electrical component.
 3. Thesystem of claim 2 wherein the control system provides power to thethermal cautery device only if the means for identifying a thermalcautery device identifies the thermal cautery device.
 4. The system ofclaim 2 wherein the identifying electrical component is a resistor. 5.The electrical circuit of claim 4 wherein the resistor is chosen fromthe group consisting of about 1 KΩ, about 10 KΩ and about 51 KΩ.
 6. Thesystem of claim 1 further comprising a capacitor placed in parallel withthe reference resistor.
 7. The system of claim 1 wherein theidentification resistor is chosen from the group consisting of about 1KΩ, about 10 KΩ and about 51 KΩ.
 8. A system for performing thermalcautery, said system comprising: a thermal cautery device, said thermalcautery device comprising a resistive heating element mounted on meansfor pressing the resistive heating element into contact with bodytissue; and a means for providing a constant current; and a means foroperably connecting the resistive heating element to the means forproviding a constant current in order to provide a constant current tothe resistive heating element when power is applied to the system; andwherein the means for providing a constant current comprises: a powersource, wherein the resistive heating element of the thermal cauterydevice is electrically connected to the power source and to the drain ofa power MOSFET; and a sense resistor electrically connected to thesource of the power MOSFET and to ground, wherein a sense voltagedevelops across the sense resistor when power is applied to the senseresistor; and an operational amplifier, wherein the output of theoperational amplifier is electrically connected to the gate of the powerMOSFET, wherein the negative terminal of the operational amplifier iselectrically connected to the sense voltage, and wherein the positiveterminal of the operational amplifier is electrically connected to acontrol voltage such that when power applied to the system the circuitwill adjust the current until the sense voltage equals the controlvoltage.
 9. The system of claim 8 further comprising: a control systemoperable to set the control voltage based on the input of a means foridentifying the thermal cautery device.
 10. The system of claim 9wherein the control system provides power to the thermal cautery deviceonly if the means for identifying a thermal cautery device identifiesthe thermal cautery device.
 11. The system of claim 9 wherein the powersource comprises a switching power supply.
 12. The system of claim 9wherein the means for identifying the thermal cautery device is based onan identifying electrical component.
 13. The system of claim 12 whereinthe identifying electrical component is a resistor.
 14. The system ofclaim 13 wherein the resistor is chosen from the group consisting ofabout 1 KΩ, about 10 KΩ and about 51 KΩ.
 15. The system of claim 9wherein the means for identifying the thermal cautery device comprises:an identification resistor electrically connected to the thermal cauterydevice, wherein the identification resistor of the thermal cauterydevice comprises a pre-determined resistance value and is capable ofreceiving power from the switching power supply; a reference resistorplaced in series with the identification resistor when the thermalcautery device is electrically connected to the switching power supply,said reference resistor also electrically connected to ground such thatan identifying voltage develops across the reference resistor when poweris applied to the reference resistor and the identification resistor; acomparison resistor electrically connected to the switching power supplyand to ground, wherein a reference voltage develops across thecomparison resistor when power is applied to the comparison resistor; acomparator, wherein the positive terminal of the comparator iselectrically connected to the identifying voltage and the negativeterminal of the comparator is electrically connected to the referencevoltage; wherein the comparator produces an output signal indicating thepresence of a particular thermal cautery device when power is applied tothe circuit if the identifying voltage is higher than the referencevoltage; wherein the comparator output comprises the control voltage.16. The system of claim 15 further comprising a capacitor placed inparallel with the reference resistor.
 17. The system of claim 15 whereinthe identification resistor is chosen from the group consisting of about1 KΩ, about 10 KΩ and about 51 KΩ.